In a two-cycle engine, the opening and closing of intake, scavenging and exhaust ports formed in the inner surface of its cylinder are usually effected by the reciprocal motion of a piston. Fresh air is drawn into a crank chamber through the intake port upon upward movement of the piston, compressed upon downward movement of the piston, and flows into a combustion chamber through a scavenging passage and the scavenging port. Combustion gas is discharged from the combustion chamber through the exhaust port.
The fresh air contains engine oil, and this oil is also discharged through the combustion chamber and the exhaust port. A part of the oil adheres to the edge of the exhaust port facing the cylinder. As the exhaust port is exposed directly to the combustion gas having a high temperature and a high pressure, the oil is heated to form carbon which is likely to be deposited on the edge of the exhaust port. This carbon gradually grows with the lapse of the operating time, resulting in a drastic reduction in the open area of the exhaust port. As a result, the engine has a drastically lower exhaust efficiency, a lower output, and an adversely affected fuel comsumption, and is encountered with a number of other disadvantages, too.
Under these circumstances, it is an object of this invention to provide a two-cycle engine which enables the prevention of carbon in the exhaust port, and ensures a minimum reduction in the open area of the exhaust port for a long period of time.